Positive-electrode material for lithium secondary battery, secondary battery employing the same, and process for producing positive-electrode material for lithium secondary battery

ABSTRACT

A subject for the invention is to provide a positive-electrode material, which has high capacity and high output and is inhibited from suffering a decrease in output with repetitions of charge and use. 
     The invention provides a positive-electrode material for lithium secondary battery, which comprises a secondary particle of a lithium/transition metal composite oxide containing boron and/or bismuth, and wherein the atomic ratio of the sum of boron and bismuth to the sum of the metallic elements other than lithium, boron, and bismuth in a surface part of the secondary particle is from 5 times to 70 times the atomic ratio in the whole secondary particle.

This is a continuation application of U.S. application Ser. No.11/758,199, filed Jun. 5, 2007, which is a continuation application ofU.S. application Ser. No. 10/950,520, filed Sep. 28, 2004, which is acontinuation of PCT/JP03/03357 filed on Mar. 19, 2003.

FIELD OF THE INVENTION

The present invention relates to a positive-electrode material forlithium secondary battery, which comprises a lithium/transition metalcomposite oxide. The invention further relates to a process forproducing the positive-electrode material for lithium secondary batteryand a secondary battery employing the positive-electrode material forlithium secondary battery.

BACKGROUND ART

With the recent trend toward size and weight reduction in portableelectronic appliances and communication appliances, there is a desirefor secondary battery having a high output and a high energy density tobe used as a power source for these appliances. Secondary battery havingsuch features is desired also for use as automotive power sources. Inparticular, lithium secondary battery is being rapidly developed becausethey satisfy those requirements.

As positive-electrode materials for lithium secondary battery are usedlithium/transition metal composite oxides the standard compositions ofwhich are LiCoO₂, LiNiO₂, LiMn₂O₄, and the like. However, variousinvestigations are being made in order to improve batterycharacteristics.

For example, patent document 1 discloses that displacing part of thecobalt in LiCoO₂ by boron or bismuth improves charge/discharge cyclecharacteristics, while patent document 2 discloses that coating thesurface of primary particles of LiCoO₂ with boron improvescharge/discharge cycle characteristics. Furthermore, patent document 3discloses that when an element Z (Bi, B, or W) is added to LiMO₂(wherein M is Co, Ni, etc.) in such an amount as to result in an atomicratio (Z/M) of 0.1 or lower and this mixture is burned, then the elementZ melts at the boundaries among primary particles to thereby increasethe size of the primary particles, and that use of the resultantcomposite oxide as a positive-electrode material brings about animproved discharge capacity.

On the other hand, of the lithium/transition metal composite oxidesmentioned above, LiMn₂O₄ and LiNiO₂ are advantageous because they areless expensive than LiCoO₂. However, for putting these composite oxidesto practical use, it is necessary to improve the composite oxides inhigh-temperature cycle characteristics, storability, atmosphere controlduring burning/storage, safety, etc. Investigations are hence being madeon LiNi_(1-x)Mn_(x)O₂ obtained by displacing part of the nickel sites inLiNiO₂ by manganese. (e.g., non-patent documents 1 to 3) However, thereis a problem that when the manganese displacement amount is increased, asufficient capacity cannot be obtained.

-   [Patent Document 1] JP-A-4-253162-   [Patent Document 2] JP-A-4-328258-   [Patent Document 3] JP-A-8-55624-   [Non-patent Document 1] J. Mater. Chem., 6 (1996), p. 1149-   [Non-patent Document 2] J. Electrochem. Soc., 145 (1998), p. 1113-   [Non-patent Document 3] Dai 41-kai Denchi Tôrôn-kai Yokô-shû    (2000), p. 460

Lithium batteries are recently required more and more to have higherperformances. Properties including high capacity, high output, andinhibition of output from decreasing with repetitions ofcharge/discharge and use are desired to be attained in a high degree.Especially for high capacity, a positive-electrode material having ahigh bulk density is desired.

However, the positive-electrode material disclosed in patent document 2,which is a material obtained by coating the surface of primary particlesof LiCoO₂ with boron, is ineffective in sufficiently inhibiting theoutput decrease which occurs with repetitions of charge/discharge anduse. In the case of the positive-electrode materials disclosed inExamples given in patent documents 1 and 3, not only an increasedcapacity is difficult to attain because the electrode materials have alow bulk density, but also the output decrease which occurs withrepetitions of charge/discharge and use cannot be sufficientlyinhibited.

DISCLOSURE OF THE INVENTION

The present inventor made intensive investigations in view of theproblems described above. As a result, the following has been found. Alithium/transition metal composite oxide which comprises boron and/orbismuth and is in the form of secondary particle and in which boron andbismuth are present in a higher concentration in a surface part of thesecondary particle as compared with the composition of the wholeparticles has a high bulk density and a small specific surface area.Because of these properties, a high capacity is attained when thiscomposite oxide is used as a positive-electrode material. Furthermore,use of this composite material as a positive-electrode material reduceselectrode resistance and, as a result, can heighten cell output. Theinvention has been completed based on this finding.

An essential point of the invention resides in a positive-electrodematerial for lithium secondary battery, which comprises a secondaryparticle of a lithium/transition metal composite oxide containing boronand/or bismuth, and wherein the atomic ratio of the sum of boron andbismuth to the sum of the metallic elements other than lithium, boron,and bismuth in a surface part of the secondary particle is from 5 timesto 70 times the atomic ratio in the whole secondary particle.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained below in detail.

Examples of the lithium/transition metal composite oxide containingboron and/or bismuth, which constitutes the positive-electrode materialfor lithium secondary battery according to the invention, includelithium/transition metal oxides having a lamellar structure,lithium/transition metal composite oxides having a spinel structure, andthe like which each contain boron and/or bismuth. Preferred of these isa lithium/transition metal composite oxide which contains lithium in anamount exceeding the stoichiometric proportion thereof in the compositeoxide and has a composition in which the atomic ratio (b/a) between theexcess lithium (a) and the sum of boron and bismuth (b) satisfies0.1≦b/a≦5, especially 0.1≦b/a≦4. Although the reasons for this areunclear, it is thought that the boron and/or bismuth present in a higherconcentration in the surface of the secondary particle is in the form ofa composite compound with lithium and this relates to the preference ofthat composite oxide. The term “stoichiometric proportion in alithium/transition metal composite oxide” means the numeral indicatingthe molar proportion.

For example, in the case of a composite oxide obtained by incorporatingbismuth and/or boron into a lithium/transition metal composite oxidehaving a lamellar structure, which is represented by LiMn₂O₄, LiNiO₂,LiCoO₂, and the like, the composition thereof is shown by formula (4).Since the stoichiometric proportion of lithium in thislithium/transition metal composite oxide of a lamellar structure is 1,the amount of excess lithium (a), i.e., that part of the lithium amount,which is outside the stoichiometric proportion, is determined by x−1.The sum of boron and bismuth (b) is determined by v+w.

LiMBi_(x)Bi_(v)O₂  (4)

(In the formula, M represents a transition metal.)

In the case of a composite oxide obtained by incorporating bismuthand/or boron into a lithium/transition metal composite oxide having aspinel structure, which is represented by LiMn₂O₄, the compositionthereof is shown by formula (5). Since the stoichiometric proportion oflithium in this lithium/transition metal composite oxide of a spinelstructure also is 1, the amount of excess lithium (a), i.e., that partof the lithium amount, which is outside the stoichiometric proportion,is determined by x−1. The sum of boron and bismuth (b) is determined byv+w.

Li_(x)M₂Bi_(v)B_(w)O₄  (5)

(In the formula, M represents a transition metal.)

The lithium/transition metal composite oxide containing boron and/orbismuth preferably is one represented by the following formula (1).

Li_(x)M_(y)Bi_(v)B_(w)O₂  (1)

In formula (1), M represents at least one element selected fromtransition metals, alkali metals, alkaline earth metals, halogenelements, and chalcogen elements. Preferred of these are one or moreelements selected from Ni, Mn, Co, Al, Fe, Ga, Sn, V, Cr, Cu, Zn, Mg,Ti, Ge, Nb, Ta, Zr, and Ca. More preferred is Ni, Mn, or Co.

In formula (1), x is a number satisfying 0<x≦1.2, preferably 0≦x≦1.15.In case where x is too large, there is the possibility that the crystalstructure might become unstable and that the lithium secondary batteryemploying this composite oxide might have a reduced battery capacity.Symbol y is a number of generally from 0.9 to 1.1, preferably from 0.95to 1.05. Symbols v and w each are 0 or larger, preferably 0.001 orlarger, more preferably 0.002 or larger, especially preferably 0.005 orlarger. With respect to the upper limit, v and w are 0.1 or smaller,preferably 0.05 or smaller, more preferably 0.03 or smaller, especiallypreferably 0.02 or smaller. In case where v and w are too small, theeffects of the invention are not produced. In case where v and w are toolarge, there is the possibility that battery performances might beimpaired.

One of v and w may be 0. The sum of v and w is larger than 0, preferably0.001 or larger, more preferably 0.002 or larger. The sum thereof isgenerally 0.2 or smaller, preferably 0.15 or smaller, more preferably0.1 or smaller.

In the composition represented by formula (1), the oxygen amount may beslightly nonstoichiometric.

Preferred of the composite oxides represented by formula (1) are onesrepresented by the following formula (2).

Li_(x)M¹ _(y1)M² _(y2)Bi_(v)B_(w)O₂  (2)

In formula (2), M¹ represents at least one element selected from Ni, Mn,and Co. M¹ may be of one kind or a combination of two or more kinds. M²represents at least one element selected from Ni, Mn, Co, Al, Fe, Ga,Sn, V, Cr, Cu, Zn, Mg, Ti, Ge, Nb, Ta, Zr, and Ca (the at least oneelement is often referred to as “displacing metallic element”).Preferred examples of M² are Al, Co, Fe, Mg, Ga, Ti, and Ca. M²preferably is Al, Co, or Mg, especially Al or Co, more preferably Co. M²may be of one kind or a combination of two or more kinds.

In formula (2), x, v, and w are the same as in formula (1). Symbols y1and y2 are numbers satisfying 0<y1 and 0≦y2, provided that y1+y2 isgenerally from 0.9 to 1.1, preferably from 0.95 to 1.05.

In the composition represented by formula (2), the oxygen amount may beslightly nonstoichiometric.

Preferred of the lithium/transition metal composite oxides containingboron and/or bismuth, which are represented by, formula (2) is acomposite oxide represented by the following formula (3), which has alamellar crystal structure and contains lithium, nickel, and manganese.

Li_(x)Ni_(α)Mn_(β)Q_((1-α-β))Bi_(v)B_(w)O₂  (3)

In formula (3), Q represents a metallic element which has displaced partof the Ni and Mn sites (hereinafter, such a metallic element is oftenreferred to as “displacing metallic element”). Examples of Q includetransition metals, alkali metals, alkaline earth metals, halogenelements, chalcogen elements, and the like. In particular, examplesthereof include elements such as Al, Fe, Ga, Sn, V, Cr, Co, Cu, Zn, Mg,Ti, Ge, Nb, Ta, Zr, and Ca. Preferred examples of Q are Al, Co, Fe, Mg,Ga, Ti, and Ca. Q preferably is Al, Co, or Mg, especially Al or Co, morepreferably Co. Q may be of one kind or a combination of two or morekinds.

In formula (3), x represents a number satisfying 0<x≦1.2, preferably0<x≦1.15. In case where x is too large, there is the possibility thatthe crystal structure might become unstable and that the lithiumsecondary battery employing this composite oxide might have a reducedbattery capacity.

Symbol α is a number generally satisfying preferably 0.3≦α0.7. Symbol βis preferably 0.05 or larger, especially 0.1 or larger, and ispreferably 0.6 or smaller, especially 0.45 or smaller. In case where βis too large, it is difficult to synthesize a lithium-nickel-manganesecomposite oxide of a single-phase structure. Conversely, in case where αis too large, not only the total cost is increased but also the effectsof the invention are not markedly produced.

The Ni/Mn molar ratio, α/β, is 0.7 or higher, preferably 0.8 or higher,especially preferably 0.9 or higher, and is 9 or lower, preferably 8 orlower, especially preferably 6 or lower.

The value of (1-α-β) is 0 or larger, and is 0.5 or smaller, preferably0.4 or smaller, more preferably 0.3 or smaller. Too high contents of thedisplacing metallic element Q are impracticable because this compositeoxide gives a cell electrode having a reduced capacity and because it isnecessary to use a large amount of an expensive raw material theresources for which are not abundant, especially when the transitionmetal element Q is cobalt.

Symbols v and w each are 0 or larger, preferably 0.001 or larger, morepreferably 0.002 or larger, especially preferably 0.005 or larger. Withrespect to the upper limit, v and w are 0.05 or smaller, preferably 0.03or smaller, more preferably 0.02 or smaller. In case where v and w aretoo small, the effects of the invention are not produced. In case wherev and w are too large, there is the possibility that cell electrodeperformances might be impaired. One of v and w may be 0. The sum of vand w is larger than 0, preferably 0.001 or larger, more preferably0.002 or larger. The sum thereof is generally 0.2 or smaller, preferably0.15 or smaller, more preferably 0.1 or smaller.

In the composition represented by formula (3), the oxygen amount may beslightly nonstoichiometric.

The lithium/transition metal composite oxide containing boron and/orbismuth is in the form of secondary particle. These secondary particleshow a single crystal phase and are characterized in that bismuth andboron are present in a higher concentration in a surface part of theparticles. Specifically, the atomic ratio of the sum of boron andbismuth to the sum of the metallic elements other than lithium, boron,and bismuth in a surface part of the secondary particle is from 5 timesto 70 times the atomic ratio of the sum of boron and bismuth to the sumof the metallic elements other than lithium, boron, and bismuth in thewhole secondary particle. This proportion preferably is 8 times or more,and is preferably up to 60 times, especially up to 50 times. In casewhere this proportion is too small, the effect of improving batteryperformances is low. Conversely, too large values of this proportionlead to impaired battery performances and deterioration in powderproperties including bulk density.

Compositional analysis of a surface part of the secondary particle isconducted by X-ray photoelectron spectroscopy (XPS) using AlK, as anX-ray source under the conditions of an analysis area of 0.8 mm indiameter and a takeout angle of 45 degrees. Although the range (depth)in which analysis is possible varies depending on the composition of thesecondary particle, it is generally from 0.1 nm to 50 nm. Especially inpositive-electrode materials, that range is generally from 1 nm to 10nm. Consequently, the term “surface part of secondary particle” as usedin the invention means the range in which analysis is possible underthose conditions.

The average particle diameter (average secondary-particle diameter) ofthese secondary particles is generally 1 μm or larger, preferably 4 μmor larger, and is generally 50 μm or smaller, preferably 40 μm orsmaller. Average secondary-particle diameters can be measured with aknown laser diffraction/scattering type particle size distributionanalyzer. Examples of dispersion media for use in this measurementinclude 0.1% by weight aqueous sodium hexametaphosphate solution. Theparticle diameter of the secondary particle can be regulated, forexample, by changing production conditions such as, e.g., sprayingconditions including gas/liquid ratio in the spray drying step whichwill be described later. In case where the particle diameter of thesecondary particle is too small, cycle characteristics and safety tendto decrease. In case where the particle diameter thereof is too large,internal resistance tends to become high and sufficient output is lessapt to be obtained.

The primary particles constituting the secondary particle have anaverage particle diameter (average primary-particle diameter) ofgenerally 0.01 μm or larger, preferably 0.02 μm or larger, morepreferably 0.1 μm or larger. The primary-particle diameter is generally10 μm or smaller, preferably 5 μm or smaller, more preferably 3 μm orsmaller. Average primary-particle diameters can be measured through anexamination with a scanning electron microscope (SEM). The size of theprimary particles can be regulated by changing, for example, productionconditions such as, e.g., burning temperature, burning period, andburning atmosphere. Incase where the primary-particle diameter is toosmall, side reactions and the like are apt to take place on the surfaceand, hence, cycle characteristics and other performances are apt todecrease. In case where the primary-particle diameter is too large, ratecharacteristics and capacity tend to decrease due to the inhibition oflithium diffusion, condition passage deficiency, etc.

The specific surface area of the secondary particle consisting of alithium/transition metal composite oxide containing boron and/or bismuthcannot be unconditionally specified because it varies considerablydepending on the composition and on the elements contained. However, thespecific surface area thereof is generally 0.1 m²/g or larger,preferably 0.2 m²/g or larger, more preferably 0.3 m²/g or larger. Toosmall a specific surface area is undesirable because it means that theprimary-particle diameter is large, i.e., rate characteristics andcapacity tend to decrease. On the other hand, too large specific surfaceareas also tend to result in a decrease in cycle characteristics, etc.Because of these, the specific surface area thereof is generally 8 m²/gor smaller, preferably 5 m²/g or smaller, more preferably 2 m²/g orsmaller.

Specific surface areas are measured with a known BET type specificsurface area meter for powders. Specifically, a BET one-pointmeasurement is made by the continuous flow method using nitrogen andhelium as an adsorbate gas and a carrier gas, respectively. First, apowder sample is heated and degassed at a temperature of 450° C. orlower with a mixed gas and subsequently cooled to a liquid-nitrogentemperature to adsorb a helium/nitrogen mixed gas thereonto. This sampleis heated to room temperature with water to desorb the adsorbed nitrogengas. The nitrogen gas being desorbed is detected with a thermalconductivity detector. The amount of the desorbed gas corresponding to adesorption peak is determined, and the specific surface area of thesample is calculated therefrom.

The tap density of the secondary particle according to the invention,which consist of a lithium/transition metal composite oxide containingboron and/or bismuth, is measured by a method comprising placing about 8g of the lithium/transition metal composite oxide powder in a 10-mLmeasuring cylinder, tapping (dropping) the cylinder 200 times, andmeasuring the density. The height from which the measuring cylinder isdropped is from 1 to 5 cm. The material of the receiving surface ontowhich the measuring cylinder is dropped is not particularly limited. Thedropping interval is from 50 to 500 drops per minute. The tap density ofthe particles cannot be unconditionally specified because it variesconsiderably depending on the composition and the elements contained.However, the tap density thereof is generally 0.8 g/cm³ or higher,preferably 1.6 g/cm³ or higher, more preferably 1.8 g/cm³ or higher,most preferably 2.0 g/cm³ or higher. In case where the tap densitythereof is too low, the amount of the positive-electrode material perunit volume is small and this necessitates an increased battery volumefor securing a certain energy capacity in fabricating a secondarybattery. In addition, low tap densities necessarily result in a lowenergy capacity when a battery having a reduced size is to be produced.Consequently, the higher the tap density, the more the particles arepreferred. However, the tap density of the particles is practically 3.0g/cm³ or lower, generally 2.5 g/cm³ or lower.

The secondary particle consisting of a lithium/transition metalcomposite oxide containing boron and/or bismuth can be produced by:forming into particles a raw-material mixture which comprises compoundscontaining the metallic elements to be components of the targetlithium/transition metal composite oxide and further containing boronand/or bismuth; and then burning this molding at a temperature higherthan the melting points of the boron compound and bismuth compound usedas raw materials.

As the raw materials can usually be used various compounds of theelements shown above, such as oxides, inorganic salts, e.g., carbonates,sulfates, nitrates, and phosphates, halides, and organic salts.

Examples of compounds containing lithium include inorganic lithium saltssuch as Li₂CO₃, LiNO₃, and lithium acetate; lithium hydroxides such asLiOH and LiOH.H₂O; lithium halides such as LiCl and LiI; inorganiclithium compounds such as Li₂O; organic lithium compounds such asalkyllithiums and fatty acid lithiums; and the like. Lithium compoundssoluble in solvents may also be used. Preferred of these are Li₂CO₃,LiNO₃, LiOH.H₂O, and lithium acetate. In the case where raw materialsare mixed by a wet process, it is preferred to use LiOH.H₂O. When wateris used as a dispersion medium, LiOH.H₂O shows an improved diffusionefficiency and improved homogeneity in the dispersion medium because itis water-soluble. In addition, since LiOH.H₂O is a compound containingno elements such as nitrogen and sulfur, use of it has an advantage thatharmful substances such as NO_(x) and SO_(X) do not generate therefromduring burning. Those lithium compounds may be used alone or incombination of two or more thereof.

Examples of compounds containing nickel include Ni(OH)₂, NiO, NiOOH,NiCO₃.2Ni (OH)₂.4H₂O, NiC₂O₄.2H₂O, Ni (NO₃)₂.6H₂O, NiSO₄, NiSO₄.6H₂O,fatty acid nickels, nickel halides, and the like. Preferred of these arethe compounds containing no elements such as nitrogen and sulfur, e.g.,Ni(OH)₂, NiO, NiOOH, NiCO₃.2Ni (OH)₂.4H₂O, and NiC₂O₄.2H₂O. This isbecause use of such a compound does not result in the generation ofharmful substances such as NO_(x) and SO_(X) therefrom during a burningstep. Especially preferred are Ni(OH)₂, NiO, and NiOOH from thestandpoints of easy availability as an industrial raw material and ofhigh reactivity in burning. Those nickel compounds may be used alone orin combination of two or more thereof.

Examples of compounds containing manganese include Mn₃O₄, Mn₂O₃, MnO₂,MnOOH, MnCO₃, Mn(NO₃)₂, MnSO₄, organomanganese compounds, manganesehydroxides, manganese halides, and the like. Preferred of thesemanganese compounds are Mn₂O₃, MnO₂, and Mn₃O₄ because they havevalences close to the manganese oxidation number for the final targetcomposite oxide.

Examples of compounds containing boron include boric acid, boron, boronhalides, boron carbide, boron nitride, boron oxide, organic complexes ofboron halides, organoboron compounds, alkylboric acids, boranes, and thelike. Any of these can be used as long as it is either a boron compoundhaving a melting point lower than the burning temperature to be used inproducing the lithium/transition metal composite oxide containing boronand/or bismuth or a boron compound which yields such boron compound.However, boric acid and boron oxide are preferred of those boroncompounds from the standpoints of easy availability as an industrial rawmaterial and of satisfactory handleability. Those boron compounds may beused alone or in combination of two or more thereof.

Examples of compounds containing bismuth include bismuth metal, bismuthoxides, bismuthhalides, bismuth carbide, bismuth nitride, bismuthhydroxide, bismuth chalcogenides, bismuth sulfate, bismuth nitrate, andorganobismuth compounds. Any of these can be used as long as it iseither a bismuth compound having a melting point lower than the burningtemperature to be used in producing the lithium/transition metalcomposite oxide containing boron and/or bismuth or a bismuth compoundwhich yields such bismuth compound. However, bismuth oxides arepreferred of those bismuth compounds from the standpoints of easyavailability as an industrial raw material and of satisfactoryhandleability. Especially preferred is Bi₂O₃. Those bismuth compoundsmay be used alone or in combination of two or more thereof.

The proportions of those raw materials to be mixed are suitably selectedaccording to the composition of the target lithium/transition metalcomposite oxide containing boron and/or bismuth.

The amounts of boron and bismuth may be suitably regulated while takingaccount of deficiencies caused by, e.g., volatilization during synthesisand of inclusion of the elements as impurities from raw materials.Specifically, however, it is preferred that the molar proportion ofbismuth (v) to be present in the reaction system be in the range ofgenerally 0≦v≦0.2, preferably 0.1, especially 0≦v≦0.05. Likewise, it ispreferred that the molar proportion of boron (w) to be present in thereaction system be in the range of generally 0≦w≦0.2, preferably0≦w≦0.1, especially 0≦w≦0.05. It is also preferred to mix raw materialsin such proportions that the atomic ratio (b/a) between the excesslithium (a), i.e., that part of the lithium amount which is outside thestoichiometric proportion in the lithium/transition metal compositeoxide, and the sum of boron and bismuth (b) is in the range of0.15≦b/a≦5. This is because secondary particle having a high bulkdensity can be easily produced from such a raw-material mixture.

Methods for mixing raw materials are not particularly limited, andeither a wet process or a dry process may be used. Examples thereofinclude methods using apparatus such as a ball mill, oscillating mill,or bead mill. Water-soluble raw materials, e.g., lithium hydroxide, maybe mixed as an aqueous solution with solid raw materials. Wet mixing ispreferred because more even mixing is possible and this can enhancereactivity in the later burning step.

The period of mixing cannot be unconditionally specified because itvaries depending on mixing methods. Any mixing period may be used aslong as the raw materials are evenly mixed on a particulate level. Forexample, in mixing with a ball mill (wet or dry mixing), the period isgenerally about from 1 hour to 2 days. In mixing with a bead mill (wetcontinuous process), the convection time is generally about from 0.1hour to 6 hours.

With respect to the degree of pulverization, the particle diameter ofthe raw-material particles is an index thereto. The particle diameterthereof is regulated to generally 2 μm or smaller, preferably 1 μm orsmaller, more preferably 0.5 μm or smaller. In case where the solidmatter in the dispersion medium containing raw materials in wet mixing(hereinafter this dispersion medium is often referred to as slurry) hastoo large an average particle diameter, not only reactivity in a burningstep is reduced, but also the spray drying which will be described latertends to yield dry particles having reduced sphericity, resulting in areduced final particle packing density. This tendency is pronouncedespecially when particles of 50 μm or smaller in average particlediameter are to be produced. Furthermore, to produce particles of anunnecessarily reduced size leads to an increased pulverization cost.Because of these, the average particle diameter of the solid matter inthe slurry is generally 0.01 μm or larger, preferably 0.02 μm or larger,more preferably 0.1 μm or larger.

The raw-material mixture is formed into particles. Methods for producinga particulate material are not particularly limited as long as aparticulate material can be obtained which comprises compoundscontaining the metallic elements to be components of the targetlithium/transition metal composite oxide and further containing boronand/or bismuth. For example, the particulate material can be obtainedby: a method in which a wet-process mixture of raw materials isspray-dried; a method in which a precipitate is yielded from an aqueoussolution of raw materials by coprecipitation and this precipitate isdried; a method in which a dry-process mixture of raw materials isformed into particles using a small amount of water or a binder; or thelike. Spray drying is preferred from the standpoints of the uniformity,powder flowability, and powder handleability of the particulate materialyielded, the ability to efficiently form secondary particle, etc.Examples thereof include: a method in which particles comprising acompound containing lithium and a compound containing a transition metalare produced by spray drying and these particles are mixed by a dryprocess with a compound containing boron and/or a compound containingbismuth; a method in which a particulate material is produced by spraydrying from a slurry containing a compound containing lithium, acompound containing a transition metal, and a compound containing boronand/or compound containing bismuth; and the like.

The average particle diameter of the particulate material is preferablyregulated to 50 μm or smaller, preferably 40 μm or smaller. However, toosmall particle diameters tend to be difficult to obtain. Consequently,the average particle diameter thereof is generally 4 μm or larger,preferably 5 μm or larger. In the case of producing a particulatematerial by spray drying, the particle diameter thereof can be regulatedby suitably selecting a spraying mode, rate of supplying a pressurizedgas flow, slurry feed rate, drying temperature, etc.

The particulate material comprising compounds containing the metallicelements to be components of the target lithium/transition metalcomposite oxide and further containing boron and/or bismuth is burned,whereby secondary particle formed by the sintering of primary particlescan be obtained. For the burning can be used, for example, a boxfurnace, tube furnace, tunnel kiln, rotary kiln, or the like. Theburning usually comprises three stages, i.e., heating, holding at amaximum temperature, and cooling. The second stage, i.e., holding at amaximum temperature, is not always conducted once and may be conductedin two or more steps according to purposes. The sequence of heating,holding at a maximum temperature, and cooling may be repeatedlyconducted two or more times each after a disaggregation step foreliminating aggregates to such a degree as not to destroy the secondaryparticle or after a pulverization step for pulverizing to fine particlessmaller than the primary particles or secondary particle. It is,however, noted that in the case where the pulverization step isinserted, the resultant pulverized particles are subjected, before beingsubsequently burned, to the step of forming secondary particle by, e.g.,spray drying.

In the heating stage, the internal temperature of the furnace iselevated at a heating rate of generally from 1 to 5° C./min. Althoughtoo slow heating is industrially disadvantageous because it takes muchtime, too quick heating makes the internal temperature of some furnacesnot to follow a set temperature.

In the stage of holding at a maximum temperature, the burningtemperature is generally 500° C. or higher, preferably 600° C. orhigher, more preferably 800° C. or higher. Too low temperatures tend tonecessitate a prolonged burning period for obtaining alithium/transition metal composite oxide having satisfactorycrystallinity. In contrast, in case where too high a temperature isused, the lithium/transition metal composite oxide undergoes severesintering. As a result, not only the yield inpulverization/disaggregation after the burning becomes poor, which isindustrially disadvantageous, but also a lithium/transition metalcomposite oxide having many defects such as oxygen deficiencies isyielded. Lithium secondary battery employing this lithium/transitionmetal composites oxide as a positive-electrode active material may havea reduced battery capacity or deteriorate due to the collapse of thecrystal structure, which proceeds with charge/discharge. Because ofthese, the burning temperature is generally 1,100° C. or lower,preferably 1,000° C. or lower.

The holding period in the stage of holding at a maximum temperature isselected generally from a wide range of from 1 hour to 100 hours. It is,however, preferred to use a holding period of 80 hours or shorter,especially 50 hours or shorter, for obtaining secondary particle havinga high boron and/or bismuth concentration in a surface part thereof. Inparticular, the holding period is preferably 30 hours or shorter,especially 20 hours or shorter, more preferably 15 hours or shorter.When the burning period is too long, there are cases where boron andbismuth do not come to be present in a higher concentration in thesurface of the secondary particle but come to be evenly distributedthroughout the secondary particle. In case where the burning period istoo short, a lithium/transition metal composite oxide havingsatisfactory crystallinity is difficult to obtain.

In the cooling stage, the internal temperature of the furnace is loweredat a cooling rate of generally from 0.1 to 5° C./min. Too slow coolingis industrially disadvantageous because it takes much time. Too quickcooling tends to result in poor uniformity of the target material and inaccelerated deterioration of the vessel.

The lithium/transition metal composite oxide containing boron and/orbismuth according to the invention, in particular thelithium-nickel-manganese composite oxide containing boron and/orbismuth, varies in bulk density, e.g., tap density, depending on burningatmospheres. Consequently, the atmosphere for the burning preferably isan atmosphere having an oxygen concentration of from 10 to 80% byvolume, more preferably from 10 to 50% by volume, such as air. When theoxygen concentration is too high, there is the possibility that theresultant lithium/transition metal composite oxide containing boronand/or bismuth might have a reduced bulk density.

In the invention, the particulate material formed is burned at atemperature higher than the melting point of the boron compound and/orbismuth compound used as raw materials. It was found that during theburning, the boron and/or bismuth contained in the particulate materialmelts and diffuses to the surface of the secondary particle whileaccelerating the sintering of the primary particles, and that the boronand/or bismuth thus comes to be finally present in a higherconcentration in the surface of the secondary particle. The inventionhas been achieved based on this finding. Such secondary particle cannotbe obtained even when a powdery mixture of raw materials is burned as itis without being formed into particles or when a molding formed bypress-molding a raw-material mixture is burned.

The thus-obtained lithium/transition metal composite oxide containingboron and/or bismuth is made up of secondary particle each consisting ofprimary particles densely sintered together as in fusion bonding. Thiscomposite oxide hence has a higher tap density than thelithium/transition metal composite oxides according to related-arttechniques. Because of this, the amount of a positive-electrode materialper unit volume can be further increased. When this composite oxide isused as a positive-electrode material for secondary battery, the energycapacity per unit cell volume can be increased and cell size reductionis also possible. In addition, the following is presumed. The secondaryparticle according to the invention has, formed in the surface thereof,a resistive layer containing nickel incorporated in lithium sites. Withrepetitions of charge/discharge, this resistive layer graduallydisappears to thereby gradually reduce resistance. Thus, the resistivelayer prevents output from decreasing with repetitions ofcharge/discharge.

The positive-electrode material according to the invention is used inthe positive electrode of a lithium secondary battery. A positiveelectrode generally comprises a current collector and formed thereon apositive-electrode active material layer comprising a positive-electrodematerial, a binder, and a conductive material. The positive-electrodeactive material in the invention is the positive-electrode material forlithium secondary battery described above. The positive-electrode activematerial layer is generally obtained by a method comprising forming theconstituent ingredients into a sheet and press-bonding this sheet to acurrent collector, a method comprising preparing a slurry containing theconstituent ingredients, applying it to a current collector, and dryingthe coating, or another method. The positive-electrode active materiallayer obtained through coating and drying is preferably pressed anddensified with a roller press or the like in order to heighten thepacking density of the electrode material.

The proportion of the positive-electrode material in thepositive-electrode active material layer is generally 10% by weight orhigher, preferably 30% by weight or higher, and is generally 99.9% byweight or lower, preferably 99% by weight or lower. Too high proportionsof the positive-electrode material tend to result in insufficientstrength of the positive electrode. In case where the proportion thereofis too low, there is the possibility of resulting in insufficientcapacity.

Examples of the conductive material for use in the positive electrodeinclude natural graphite, artificial graphite, acetylene black, and thelike. The proportion of the conductive material in the active materiallayer is generally 0.1% by weight or higher, preferably 1% by weight orhigher, and is generally 50% by weight or lower, preferably 10% byweight or lower. Too high proportions of the conductive material mayresult in insufficient capacity, while too low proportions thereof mayresult in insufficient electrical conductivity.

Examples of the binder for use in the positive electrode includepoly(vinylidene fluoride), polytetrafluoroethylene, polyvinyl acetate),poly(methylmethacrylate), polyethylene, nitrocellulose, and the like.The proportion of the binder in the positive-electrode active materiallayer is generally 0.1% by weight or higher, preferably 1% by weight orhigher, and is generally 60% by weight or lower, preferably 40% byweight or lower. Too high proportions thereof may result in insufficientcapacity, while too low proportions thereof may result in insufficientstrength.

Examples of solvents usable in preparing the slurry for use in formingthe positive-electrode active material layer includeN-methylpyrrolidone, tetrahydrofuran, dimethylformamide, water, and thelike. Examples of the material of the positive-electrode currentcollector include aluminum, stainless steel, and the like. Aluminum ispreferred.

The lithium secondary battery according to the invention generally hasthe positive electrode described above, a negative electrode, and anelectrolyte.

As the negative electrode may be used one comprising a current collectorand formed thereon a negative-electrode active material layer comprisinga negative-electrode active material and a binder and optionallycontaining a conductive material. Also usable as the negative electrodeis a foil of a metal such as lithium metal or a lithium alloy, e.g., alithium-aluminum alloy.

As the negative-electrode active material is preferably used a carbonmaterial. Examples of the carbon material include natural graphite,pyrolysis carbon, and the like. The material of the negative-electrodecurrent collector preferably is copper. Examples of the binder andconductive material for use in the negative electrode include the samebinders and conductive materials as those for use in the positiveelectrode. A preferred negative electrode comprises a current collectorand formed thereon a negative-electrode active layer containing a carbonmaterial.

Examples of the electrolyte include electrolytic solutions, solidelectrolytes, gel-form electrolytes, and the like. Preferred areelectrolytic solutions, in particular, nonaqueous electrolyticsolutions. Examples of the nonaqueous electrolytic solutions includeones prepared by dissolving various electrolyte salts in nonaqueoussolvents. Examples of the electrolyte salts include lithium salts suchas LiCiO₄, LiAsF₆, LiPF₆, LiBF₄, LiBr, and LiCF₃SO₃.

Examples of the nonaqueous solvents include tetrahydrofuran,1,4-dioxane, dimethylformamide, acetonitrile, benzonitrile, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate, ethylenecarbonate, propylene carbonate, butylene carbonate, and the like. Theseelectrolyte salts and nonaqueous solvents may be used alone or as amixture of two or more thereof.

A separator is generally disposed between the positive electrode and thenegative electrode. Examples of the separator include microporouspolymer films made of a polymer such as polytetrafluoroethylene,polyethylene, polypropylene, or a polyester, nonwoven-fabric filtersmade of glass fibers or the like, composite nonwoven-fabric filters madeof glass fibers and polymer fibers, and the like.

EXAMPLES

The invention will be explained below in more detail by means ofExamples, but the invention should not be construed as being limited tothe following Examples unless the invention departs from the spiritthereof.

<Battery Evaluation> <A. Production of Positive Electrode and CapacityExamination>

The lithium-nickel-manganese composite oxide obtained in each of theExamples and Comparative Examples which will be given later wassufficiently mixed in an amount of 75 parts by weight with 20 parts byweight of acetylene black and 5 parts by weight of apolytetrafluoroethylene powder using a mortar. This mixture was formedinto a thin sheet, which was punched with 9-mmφ and 12-mmφ punches. Inthis operation, the sheet was regulated so that the whole disks thuspunched out had weights of about 8 mg and about 18 mg, respectively.These disks were press-bonded to an expanded metal made of aluminum.Thus, positive electrodes were obtained.

A coin cell was fabricated using the positive electrode punched out in 9mmφ as a test electrode and using lithium metal as the counterelectrode. This cell was charged at a constant current of 0.2 mA/cm² to4.2 V (reaction for causing the positive electrode to release lithiumions) and then discharged at a constant current of 0.2 mA/cm² to 3.0 V(reaction for causing the positive electrode to occlude lithium ions).The initial charge/discharge capacity E [%] in this operation wasdetermined using the equation E [%]=Qs(D)/Qs(C), wherein Qs(C) [mAh/g]was the initial charge capacity per unit weight of thepositive-electrode active material and Qs(D) [mAh/g] was the initialdischarge capacity per unit weight thereof.

<B. Production of Negative Electrode and Capacity Examination>

A graphite powder having an average particle diameter of about from 8 to10 μm (d002=3.35 Å) was mixed in an amount of 92.5 parts by weight with7.5 parts by weight of poly(vinylidene fluoride). To this mixture wasadded N-methylpyrrolidone to prepare a slurry. This slurry was appliedto one side of a copper foil having a thickness of 20 μm, and thecoating was dried to remove the solvent. Thereafter, a 12-mmφ disk waspunched out thereof and pressed at 0.5 ton/cm² to produce a negativeelectrode.

A cell was fabricated using this negative electrode as a test electrodeand lithium metal as the counter electrode. The negative electrode wascaused to occlude lithium ions at a constant current of 0.5 mA/cm² until0 V. The initial occlusion capacity per unit weight of thenegative-electrode active material in this operation is expressed by Qf[mAh/g].

<C. Fabrication of Coin Cell and Battery Performance Evaluation>

The positive electrode punched out in 12 mmφ was placed on apositive-electrode can. A porous polyethylene film having a thickness of25 μm was placed as a separator on the positive electrode. After thesecomponents were pressed with a gasket made of polypropylene, thenegative electrode was placed thereon. A spacer for thickness regulationwas placed. Thereafter, a nonaqueous electrolytic solution wasintroduced into the coin cell and sufficiently infiltrated. Anegative-electrode can was then put thereon and the coin cell wassealed.

As the nonaqueous electrolytic solution was used one prepared bydissolving lithium hexafluorophosphate (LiPF₆) in a concentration of 1mol/L in a mixed solvent consisting of ethylene carbonate (EC) anddiethyl carbonate (DEC) in a volume ratio of 3:7.

The balance between the weight of the positive-electrode active materialand the weight of the negative-electrode active material was regulatedso as to almost satisfy the following equation.

(Weight of positive-electrode active material [g])/(weight ofnegative-electrode active material [g])=(Qf[mAh/g]/1.2)/Qs(C)[mAh/g]

<D. Cycle Test>

The 1-hour-rate current value (1 C) for the cell was set as shown by thefollowing equation, and the following measurement was made.

1 C [ma]=Qs(D)×(weight of positive-electrode active material [g])

First, at room temperature, 2 cycles of charge/discharge at a constantcurrent of 0.2 C and 1 cycle of charge/discharge at a constant currentof 1 C were conducted. Subsequently, a test consisting of 1 cycle ofcharge/discharge at a constant current of 0.2 C and subsequent 100cycles of charge/discharge at a constant current of 1 C was conducted atan elevated temperature of 60° C. The upper limit in charge was 4.1 Vand the lower-limit voltage was 3.0 V.

Through this operation, the high-temperature cycle capacity retention P[%] was determined using the following equation, wherein Qh(1) was thedischarge capacity for the first cycle in the test stage consisting of100 cycles of 1-C charge/discharge at 60° C. and Qh(100) was thedischarge capacity for the hundredth cycle in that stage.

P [%]={Qh(100)/Qh(1)}×100

<E. Measurement of Room-Temperature Resistance>

Conditioning current value I [mA]=(Qs(D) [mAh/g])×(weight ofpositive-electrode active material M [g])/5

The coin cell obtained was subjected to initial conditioning consistingof 2 cycles of charge/discharge at the conditioning current value Idetermined with the equation given above and at a charge upper-limitvoltage of 4.1 V and a discharge lower-limit voltage of 3.0 V. Thedischarge capacity Qs₂ (D) [mAh/g] per unit weight of thepositive-electrode active material in the second cycle in this operationwas measured.

The value of 1 C was set based on the following equation, and thefollowing measurement was made.

1 C [mA]=(Qs₂(D) [mAh/g])×(weight of positive-electrode active materialM [g])

The battery was sufficiently relaxed in a 25° C. room-temperatureatmosphere. Thereafter, the battery was charged at a constant current of⅓ C [mA] for 108 minutes, allowed to stand for 1 hour, and thendischarged at a constant current of 3 C for 10 seconds.

The difference ΔV [mV] between the voltage as measured after the10-second discharge V [mV] and the voltage as measured before thedischarge V₀ [mV], i.e., V [mV]−V₀ [mV], was calculated. From thedischarge current of 3 C [mA], the resistance R [Ω] was calculated usingthe equation R [Ω]=ΔV [mV]/3 C [mA]. The smaller the value of thisresistance R [Ω], the higher the effects of attaining excellent chargecharacteristics at room temperature, suitability for rapid charge, etc.

This resistance measurement was made before and after the <D. CycleTest> to determine the values of room-temperature resistance before andafter the cycle test. The change ratio between these was alsodetermined.

Example 1

LiOH.H₂O, NiO, Mn₂O₃, Co(OH)₂, and H₃BO₃ were mixed together so as toresult in Li:Ni:Mn:Co:B=1.05:0.65:0.15:0.20:0.010 (molar ratio). Purewater was added thereto to prepare a slurry having a solid concentrationof 25% by weight. This slurry was treated with a circulating wetpulverizer of the medium stirring type to pulverize until the averageparticle diameter of the solid matter in the slurry became 0.30 μm.Thereafter, the slurry was spray-dried with a four-fluid-nozzle typespray dryer to obtain a particulate material. During the spray drying,air was used as a drying gas. The drying gas introduction rate was 1,200L/min and the drying gas inlet temperature was 90° C.

About 8 g of the particulate material obtained by the spray drying wasintroduced into an alumina crucible having a diameter of 50 mm. Thiscrucible was placed in an atmospheric burning furnace. While air wasbeing passed at a flow rate of 9 L/min, the particulate material washeated at a heating rate of 5° C./min to a maximum temperature of 830°C., held at 830° C. for 10 hours, and then cooled at a cooling rate of5° C./min. Thus, a lithium-nickel-manganese composite oxide (averageparticle diameter, 8 μm) having almost the same molar composition as inthe raw materials fed was obtained. From an X-ray powder diffractionpattern, this lithium-nickel-manganese composite oxide was ascertainedto have a single-phase lamellar structure.

About 5 g of the lithium-nickel-manganese composite oxide obtained wasplaced in a 10-mL measuring cylinder made of glass. After this measuringcylinder was tapped 200 times, the powder packing density (tap density)was measured. As a result, the density was found to be 1.89 g/cc.

This composite oxide was examined for BET specific surface area with“Fully Automatic Powder Specific Surface Area Meter, Type AMS8000”manufactured by Okura Riken. As a result, the specific surface areathereof was found to be 0.70 m²/g.

Furthermore, the surface of secondary particle of this composite oxidewas subjected to compositional analysis by X-ray photoelectronspectroscopy (XPS) (X-ray photoelectron spectrophotometer “ESCA-5500MC”manufactured by Physical Electronics; X-ray source, AlK_(α); analysisarea, 0.8 mm in diameter; takeout angle, 45 degrees). As a result, theatomic ratio of boron (B/(Ni₊Mn₊Co)) in the surface of the secondaryparticle was found to be 32 times the atomic ratio of boron in the wholesecondary particle.

The lithium-nickel-manganese composite oxide obtained was used tofabricate a lithium secondary battery, which was evaluated. The resultsare shown in Table 1.

Example 2

A single-phase lithium-nickel-manganese composite oxide was obtained inthe same manner as in Example 1, except that LiOH.H₂O, NiO, Mn₂O₃,Co(OH)₂, and Bi₂O₃ were used as raw materials in such proportions as toresult in Li:Ni:Mn:Co:Bi 1.05:0.65:0.15:0.20:0.020 (molar ratio).

This composite oxide was examined for various properties in the samemanner as in Example 1. As a result, the particles were found to have apowder packing density (tap density) of 1.90 g/cc and a BET specificsurface area of 0.60 m²/g. The atomic ratio of bismuth (Bi/(Ni⁺ Mn+Co))in the surface of the secondary particle was found to be 9 times theatomic ratio of bismuth in the whole secondary particle.

The lithium-nickel-manganese composite oxide obtained was used tofabricate a lithium secondary battery, which was evaluated. The resultsare shown in Table 1.

Comparative Example 1

A lithium-nickel-manganese composite oxide was obtained in the samemanner as in Example 1, except that boric acid was not added. From anX-ray powder diffraction pattern, the lithium-nickel-manganese compositeoxide obtained was ascertained to have a single-phase lamellarstructure.

This composite oxide was examined for various properties in the samemanner as in Example 1. As a result, the particles were found to have apowder packing density (tap density) of 1.73 g/cc and a BET specificsurface area of 0.69 m²/g.

The lithium-nickel-manganese composite oxide obtained was used tofabricate a lithium secondary battery, which was evaluated. The resultsare shown in Table 1.

Example 3

A single-phase lithium-nickel-manganese composite oxide was obtained inthe same manner as in Example 1, except that LiOH.H₂O, NiO, Mn₂O₃,Co(OH)₂, and Bi₂O₃ were used as raw materials in such proportions as toresult in Li:Ni:Mn:Co:B=1.05:0.33:0.33:0.33:0.005 (molar ratio), andthat the maximum temperature in the burning was changed to 900° C.

This composite oxide was examined for various properties in the samemanner as in Example 1. As a result, the particles were found to have apowder packing density (tap density) of 1.70 g/cc and a BET specificsurface area of 0.80 m²/g. The atomic ratio of bismuth (Bi/(Ni+Mn+Co))in the surface of the secondary particle was found to be 44 times theatomic ratio of bismuth in the whole secondary particle.

The lithium-nickel-manganese composite oxide obtained was used tofabricate a lithium secondary battery, which was evaluated. The resultsare shown in Table 1.

Comparative Example 2

A lithium-nickel-manganese composite oxide was obtained in the samemanner as in Example 3, except that bismuth was not added. From an X-raypowder diffraction pattern, the lithium-nickel-manganese composite oxideobtained was ascertained to have a single-phase lamellar structure.

This composite oxide was examined for various properties in the samemanner as in Example 1. As a result, the particles were found to have apowder packing density (tap density) of 1.01 g/cc and a BET specificsurface area of 2.27 m²/g.

The lithium-nickel-manganese composite oxide obtained was used tofabricate a lithium secondary battery, which was evaluated. The resultsare shown in Table 1.

Reference Example 1

A lithium-nickel-cobalt-aluminum composite oxide was obtained in thesame manner as in Example 1, except that LiOH.H₂O, NiO, Co(OH)₂, Al₂O₃,and H₃BO₃ were used in such proportions as to result inLi:Ni:Co:Al:B=1.05:0.82:0.15:0.03:0.01 (molar ratio). From an X-raypowder diffraction pattern, the powder obtained was ascertained to havea single-phase lamellar structure.

This composite oxide was examined for powder packing density (tapdensity) in the same manner as in Example 1. As a result, the tapdensity thereof was found to be 1.62 g/cc.

Reference Example 2

A lithium-nickel-cobalt-aluminum composite oxide was obtained in thesame manner as in Reference Example 1, except that boron was not added.From an X-ray powder diffraction pattern, the powder obtained wasascertained to have a single-phase lamellar structure.

This composite oxide was examined for powder packing density (tapdensity) in the same manner as in Example 1. Asa result, the tap densitythereof was found to be 1.77 g/cc.

TABLE 1 Composition of positive- Tap density Specific surface areaelectrode material b/a (g/cm³) (m²/g) *1 Example 1Li_(1.05)Ni_(0.65)Mn_(0.15)Co_(0.20)B_(0.01)O₂ 0.2 1.89 0.70 32 Example2 Li_(1.05)Ni_(0.65)Mn_(0.15)Co_(0.20)Bi_(0.02)O₂ 0.4 1.90 0.64 9Comparative Li_(1.05)Ni_(0.65)Mn_(0.15)Co_(0.20)O₂ 0 1.73 0.69 — Example1 Example 3 Li_(1.05)Ni_(0.33)Mn_(0.33)Co_(0.33)Bi_(0.005)O₂ 0.1 1.700.85 44 Comparative Li_(1.05)Ni_(0.33)Mn_(0.33)Co_(0.33)O₂ 0 1.01 2.27 —Example 2 Reference Li_(1.05)Ni_(0.82)Co_(0.15)Al_(0.03)Bi_(0.01)O₂ 0.21.62 Example 1 Reference Li_(1.05)Ni_(0.82)Co_(0.15)Al_(0.03)O₂ 0 1.77Example 2 Initial Cycle charge/discharge capacity Room-temperatureresistance efficiency retention Before cycle test After cycle testChange ratio E (%) P (%) R1 (Ω) R2(Ω) (R2/R1) Example 1 87.0 87.6 14.621.3 1.45 Example 2 88.9 87.2 13.7 17.1 1.25 Comparative 84.8 87.9 19.040.8 2.15 Example 1 Example 3 89.7 81.4 16.3 16.4 1.01 Comparative 90.086.7 12.2 14.9 1.22 Example 2 *1: Proportion of the atomic ratio of thesum of boron and bismuth to the sum of the metallic elements other thanlithium, boron, and bismuth in surface part of secondary particle to theatomic ratio in the whole secondary particle.

A comparison of Examples 1 and 2 with Comparative Example 1 in Table 1shows the following. The lithium-nickel-manganese composite oxides ofthe invention, in which boron or bismuth is present in the surface ofthe secondary particle in a high concentration which is from 5 times to70 times the concentration thereof in the positive-electrode activematerial composition, have a higher tap density and attain a lowerchange ratio with respect to the change in coin cell room-temperatureresistance through the 100-cycle charge/discharge test at 60° C. thanthe lithium-nickel-manganese composite oxide which contains neitherboron nor bismuth and has the same composition with respect to theproportions of the other metals.

The composite oxides of Examples 1 and 2 are thus found to beadvantageous when used as a positive-electrode material for lithiumsecondary battery. A comparison between Example 3 and ComparativeExample 2 shows that the composite oxide of Example 3 likewise has ahigher tap density and attains a lower change ratio with respect to thechange in coin cell room-temperature resistance through the 100-cyclecharge/discharge test at 60° C. It can be seen that in compositionshaving high manganese content as in Example 3, a high bulk density couldbe realized by causing boron and/or bismuth to be present in the surfaceof the secondary particle in a higher concentration from 5 to 70 timesthe concentration thereof in the positive-electrode active materialcomposition.

This could be attained even in the composition of Comparative Example 3,which had an exceedingly low bulk density. It can be seen that incompositions having high nickel content as in the Reference Examples,the effects of the invention are low.

It was ascertained from Table 1 that the lithium-nickel-manganesecomposite oxides obtained in the Examples have a high powder tap densityand a small specific surface area and give a secondary battery havingexcellent battery performances.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on a Japanese patent application filed on Mar.28, 2002 (Application No. 2002-091473), the contents thereof beingherein incorporated by reference.

INDUSTRIAL APPLICABILITY

According to the invention, a positive-electrode material for lithiumsecondary battery having high performances (high capacity, high cyclecharacteristics, high rate characteristics, high storability, etc.)which is suitable for use as a positive-electrode material for lithiumsecondary battery can be provided at low cost. In particular, alithium/transition metal composite oxide having a higher bulk densitythan those, which have been synthesized hitherto, can be provided by theinvention. Furthermore, the invention can provide a positive electrodefor lithium secondary battery having high performances (highcharge/discharge efficiency, low resistance and high output, etc.) and alithium secondary battery.

1. A positive-electrode material for lithium secondary battery, whichcomprises secondary particle of a lithium/transition metal compositeoxide containing boron and/or bismuth, and wherein the atomic ratio ofthe sum of boron and bismuth to the sum of the metallic elements otherthan lithium, boron, and bismuth in a surface part of the secondaryparticle is 8 times or more the atomic ratio in the whole secondaryparticle.
 2. The positive-electrode material for lithium secondarybattery according to claim 1, wherein the lithium/transition metalcomposite oxide containing boron and/or bismuth contains excess lithiumbased on the stoichiometric proportion thereof in the lithium/transitionmetal composite oxide, and wherein the atomic ratio (b/a) between theexcess lithium based on the stoichiometric proportion (a) and the sum ofboron and bismuth (b) is from 0.1 to
 5. 3. The positive-electrodematerial for lithium secondary battery according to claim 1 or 2,wherein the lithium/transition metal compound containing boron and/orbismuth is one represented by the following formula (2):Li_(x)M¹ _(y1)M² _(y2)Bi_(v)B_(w)O₂  (2) wherein M¹ represents at leastone element selected from Ni, Mn, and Co; M² represents at least oneelement selected from Ni, Mn, Co, Al, Fe, Ga, Sn, V, Cr, Cu, Zn, Mg, Ti,Ge, Nb, Ta, Zr, and Ca; and x, y1, y2, v, and w respectively representnumbers satisfying the relationships 0<x≦1.2, 0<y1, 0≦y2, 0.9≦y1+y2≦1.1,and 0≦w≦0.1 (provided that at least one of v and w is not 0).
 4. Thepositive-electrode material for the secondary lithium battery accordingto claim 1 or 2, wherein the lithium/transition metal composite oxidecontaining boron and/or bismuth is one represented by the followingformula (3):Li_(x)Ni_(α)Mn_(β)Q_((1-α-β))Bi_(v)B₄O₂  (3) wherein Q represents atleast one element selected from the group consisting of Al, Fe, Ga, Sn,V, Cr, Co, Cu, Zn, Mg, Ti, Ge, Nb, Ta, Zr, and Ca; and x, α, β, v, and wrespectively represent numbers satisfying the relationships 0<x≦1.2,0.7≦α/β≦9, 0≦(1-α-β)≦0.5, 0≦v≦0.1, and 0≦w≦0.1 (provided that at leastone of v and w is not 0).
 5. The positive-electrode material for lithiumsecondary battery according to claim 4, wherein a satisfies 0.3≦α≦0.8.6. The positive-electrode material for lithium secondary batteryaccording to claim 4, wherein β satisfies 0.05≦β≦0.6.
 7. Thepositive-electrode material for lithium secondary battery according toclaim 1 or 2, wherein the secondary particle of a lithium/transitionmetal composite oxide containing boron and/or bismuth has a specificsurface area of from 0.1 m²/g to 8 m²/g.
 8. The positive-electrodematerial for lithium secondary battery according to claim 1 or 2,wherein the secondary particle of a lithium/transition metal compositeoxide containing boron and/or bismuth has a tap density of from 0.8g/cm³ to 3.0 g/cm³.
 9. The positive-electrode material for lithiumsecondary battery according to claim 1 or 2, wherein the secondaryparticle of a lithium/transition metal composite oxide containing boronand/or bismuth has an average particle diameter of from 1 μm to 50 μm.10. A positive electrode for lithium secondary battery, which comprisesthe positive-electrode material for lithium secondary battery accordingto claim 1 or 2 and a binder.
 11. A lithium secondary battery, whichcomprises the positive electrode for lithium secondary battery accordingto claim 10, a negative electrode, and an electrolyte.
 12. A process forproducing the positive-electrode material for lithium secondary batteryaccording to claim 1 or 2, wherein a raw-material mixture whichcomprises compounds containing the metallic elements to be components ofthe target lithium/transition metal composite oxide and furthercontaining boron and/or bismuth is formed into particles and thismolding is burned at a temperature higher than the melting points of theboron compound and bismuth compound used as raw materials to therebyyield secondary particle of the lithium/transition metal composite oxidecontaining boron and/or bismuth.
 13. The process for producing apositive-electrode material for lithium secondary battery according toclaim 12, wherein the burning is conducted in an atmosphere having anoxygen concentration of from 10 to 80% by volume.
 14. The process forproducing a positive-electrode material for lithium secondary batteryaccording to claim 12, wherein the boron source comprises at least onemember selected from the group consisting of boric acid, boron, boronhalides, boron carbide, boron nitride, boron oxide, organic complexes ofboron halides, organoboron compounds, alkylboric acids, and boranes. 15.The process for producing a positive-electrode material for lithiumsecondary battery according to claim 12, wherein the bismuth sourcecomprises at least one member selected from the group consisting ofbismuth metal, bismuth oxides, bismuth halides, bismuth carbide, bismuthnitride, bismuth hydroxide, bismuth chalcogenides, bismuth sulfate,bismuth nitrate, and organobismuth compounds.